It is known to provide an implosion-proof cathode-ray tube by means of a metal ring which is comprised of at least one band-like metal strip member connected at least at one part and which is fitted by a so-called shrinkage fit on the maximum peripheral length part, or the so-called mold match line formation part, of the cathode-ray tube panel.
The aforementioned reinforcement shrinkage fit is briefly explained. There is prepared a metal ring having an inner peripheral length less than the peripheral length of the mold match line part of the cathode-ray tube, which represents the maximum peripheral length part of the tube, at least one connecting portion formed by caulking or welding, and a shape similar to the mold match line part. This metal ring is heated or baked so that the inner peripheral length of the ring is equal to or slightly larger than the outer peripheral length of the mold match line part owing to the resulting thermal expansion of the ring. The mold match line part of the cathode-ray tube is provided with a viscous adhesive layer such as an adhesive tape about which the metal ring is fitted after it is heated and expanded in the above described manner. The metal ring is allowed to cool to be contracted so that a clamping tension of the metal ring is developed so as to assure an implosion-proof reinforcement of the metal ring.
This clamping tension is determined as the strain or deformation .DELTA.l which is the difference between the original peripheral length l of the metal ring (the length prior to fitting of the metal ring on the cathode-ray tube) and the peripheral length thereof after fitting of the ring on the cathode-ray tube.
FIG. 1 shows a strain-stress curve for the metal ring which is prepared by forming a band-like strip member 1.2 mm thick and 20 mm wide to an annular body having a peripheral length l approximately equal to 1141 mm and a shape similar the mold match line part. In this figure, the ratio of strain or deformation .DELTA.l to the peripheral length l (.DELTA.l/l) in percent (%) (.DELTA.l/l.times.100%) is plotted on the horizontal axis and the stress in kg/mm.sup.2 is plotted on the vertical axis. The area of the strain between 0% and e.g. 0.13% is an elastic area in which the stress is changed in proportion to the strain. The area of plastic deformation is the area in which the strain is increased to higher than that applied in the elastic area. Within this plastic deformation area, there is a sub-area in which the strain is changed but the stress remains constant. It is this sub-area which is useful for providing the constant clamping tension of the metal ring.
On the other hand, it is not always possible to make the outer peripheral length of the mold match line part of the cathode-ray tube or the inner peripheral length of the metal ring constant because of possible dimensional errors. For example, the outer peripheral length of the mold match line part is necessarily changed within the manufacture tolerances with difference in the metal dies or press lots or wear caused to the press dies. It is therefore necessary that the dimensional errors be reduced to the smallest value possible in order that the strain be confined within the above described useful area. In the example of FIG. 1, the range of strain from about 0.13% to about 0.9% represents the useful area for which the stress remains constant at approximately 30 kg/mm.sup.2. The method of manufacture of the metal ring so far employed for reducing the manufacture or dimensional error of the mold match line part of the cathode-ray tube is as follows: The band-like steel strip member, for example, is previously machined substantially to the shape of the mold match line part of the cathode-ray tube by forming or press working. The part 1 such as the band-like steel strip member machined in the above described manner is placed about a mold 2 having a peripheral length shorter by a constant length than the outer peripheral length of the mold match line part, and pulled with a force F. Alternatively, the part 1 is wound about a form 2 designed for setting the peripheral length of the metal ring, and then it is pressed with a force F by a press jig 3. The parts 1 shown in FIG. 2 or 3, the peripheral length and shape of which are previously determined in the above described manner, are welded at least one portions thereof by a back-side welding electrode 4 and a front-side welding electrode 5 for providing a completed metal ring.
In the conventional manufacture method shown in FIGS. 2 and 3, the parts such as steel strip members are prestressed in tension during the machining step which determines the peripheral length or shape of the metal ring, and the parts are connected together in this state by welding. It is therefore necessary that the springback to which the steel strip member is subject be taken into account in setting the equipment conditions. The springback of the steel band etc. tends to be changed as a function of the fluctuations in the tension applied to the steel strip member so that it is extremely difficult to reduce the tolerance of the peripheral length of the metal ring to a smaller value.
It is also known that the parts formed or press worked in the above described manner are connected together at least at one point thereof to an annular body by a welding operation, after which a force is applied from the inside of the annular body for expanding it to the plastic deformation area so as to assure the desired inner peripheral length. However, it is difficult to elevate machining accuracy of the inner peripheral length of the metal ring because of the fluctuations in the peripheral length caused during the previous welding operation or the fluctuations in the springback caused at the time of expanding the metal ring.
The present invention has been made in consideration of the above described status of the prior art and contemplates to provide a cathode-ray tube implosion-preventing metal ring which is simple in construction, easy to manufacture and capable of reducing the dimensional error of the peripheral length of the metal ring to a smaller value.